A multi-chamber container for an infusion is known, which is provided with a medical bag formed with a flexible or soft film and having opposed faces, which are welded at a relatively low temperature to form a weak seal (partition wall) for creating a plurality of compartments or cells for respective storage of different medical liquids. At the outer periphery of the medial bag, an outlet port as a plastic molded product is provided, which outlet port forms a tubular shape having an inner space provided with a first end opened to one of the compartments and a second end fitted with a rubber plug. Prior to giving the medical liquids to a patient, the medical bag is subjected to a pressing from its outside, so that the weak seal is separated and opened, causing the space inside the bag to be unified, resulting in a mixing of the medical liquids. Thus, a piecing of the rubber plug by a needle of an infusion unit allows the medical liquids to be given. In short, in this mixing type of container for medical use, an operation for opening the weak seal for obtaining the mixing of medical liquids is essential prior to the commencement of an administration. By a piercing of the rubber plug without opening the weak seal, an erroneous operation is likely that an administration of medical liquid only at the compartment adjacent the outlet port is done. In order to combat this problem, an improved construction of an outlet port has been proposed, wherein the outlet port has a breakable end wall in the medical bag, from which breakable end wall stress imparting parts are integrally extended in a manner that the stress imparting parts are firmly welded to the respective opposed inner surfaces of the medical bag. The stress imparting parts are opened in cooperation with an inflated deformation of the medical bag as obtained when opening the compartments in a manner that a breakage of the end wall of the outlet port occurs, which causes the outlet port to be connected with the space inside the medical bag. See patent publication No. 1.    Patent Publication No. 1: Japanese Un-examined Patent Publication No. 2006-87904